420 FRAGMENTS 



the water becomes a center of disturbance, from which u 

 series of ring-ripples expand outward. Gravity and inertia 

 are the agents by which this wave-motion is produced, and 

 a rough experiment will suffice to show that the rate of 

 propagation does not amount to a foot a second. A series 

 of slight mechanical shocks is experienced by a body plunged 

 in the water, as the wavelets reach it in succession. But a 

 finer motion is at the same time set up and propagated. If 

 the head and ears be immersed in the water, as in an exper- 

 iment of Franklin's, the tick of the drop is heard. Now, 

 tins sonorous impulse is propagated, not at the rate of a 

 foot, but at the rate of 4,700 feet a second. In this case 

 it is not the gravity but the elasticity of the water that 

 comes into play. Every liquid particle pushed against its 

 neighbor delivers up its motion with extreme rapidity, 

 and the pulse is propagated as a thrill. The incom- 

 pressibility of water, as illustrated by the famous Floren- 

 tine experiment, is a measure of its elasticity; and to 

 the possession of this property, in so high a degree, the 

 rapid transmission of a sound-pulse through water is to be 

 ascribed. 



But water, as you know, is not necessary to the con- 

 duction of sound; air is its most common vehicle. And 

 you know that when the air possesses the particular density 

 and elasticity corresponding to the temperature of freezing 

 water, the velocity of sound in it is 1,090 feet a second. 

 It is almost exactly one-fourth of the velocity in water; 

 the reason being that though the greater weight of the 

 water tends to diminish the velocity, the enormous molec- 

 ular elasticity of the liquid far more than atones for the 

 disadvantage due to weight. By various contrivances we 

 can compel the vibrations of the air to declare themselves; 

 we know the length and frequency of the sonorous waves, 

 and we have also obtained great mastery over the various 

 methods by which the air is thrown into vibration. We 

 know the phenomena and laws of vibrating rods, of organ- 

 pipes, strings, membranes, plates, and bells. We can 

 abolish one sound by another. We know the physical 

 meaning of music and noise, of harmony and discord. In 

 short, as regards sound in general, we have a very clear 

 notion of the external physical processes which correspond 

 to our sensations. 



In the phenomena of sound, we travel a very little way 



